The proposed studies pertain to factors which might underlie the development and maturation of neuronal circuitry in the cerebrum and cerebellum. Certain regions within the brainstem (i.e., thalamic nuclei) receive convergent inputs from the forebrain and hindbrain cortices, and in turn through polysynaptic relays, reciprocally project back onto these same cortical areas. Other muclei within the brainstem (i.e., locus coeruleus and dorsal raphe) give rise to extremely divergent axonal projections that might interrelate the ontogeny and maturation of cerebral and cerebellar circuits by way of a monosynaptic innervation of both structures. The possible branched nature of these monoaminergic pathways will be assessed in mice following forebrain and cerebellar injections utilizing three different double retrograde axonal tracing strategies: (1) horseradish peroxidase and radiolabeled lectins. (2) certain fluorescent markers; and (3) horseradish peroxidase and iron dextran. The existence of "double labeled" branched neurons with locus coeruleus and dorsal raphe will be light microscopically analyzed following autoradiography, histochemistry, and fluorescence microscopy demonstrating the transported horseradish peroxidase, lectins, iron dextran and fluorescent labels in single neurons. Distributions of branched neurons within these nuclein will be evaluated as to their possible role in interrelating areas of the forebrain and cerebellum that have similar functions or somatotopy. The data from the proposed studies might implicate particular regions of the brainstem as being involved in interrelating the development and function of cerebral-cerebellar circuits that ultimately lead to the generation of appropriate adult somatic, sensory, motor and emotional behaviors.